Semiconductor components are used in portable applications such as mobile telephones, portable computers, calculators, cameras, Personal Digital Assistants (PDAs), video game controllers, etc. and in non-portable applications such as mainframe computers, test equipment, automotive, communications, manufacturing, etc. In some of these applications it may be desirable for the semiconductor components to drive a current sensitive load or device such as, for example, a Light Emitting Diode (LED). An LED is referred to as a current sensitive device because the brightness of the LED is controlled by the amount of current flowing through the LED. A common technique for driving an LED involves the use of a Pulse Width Modulation (PWM) signal. FIG. 1 illustrates an example of an LED load driven by a PWM signal. What is shown in FIG. 1 is a circuit schematic of a prior art driver circuit 10 operable to drive an LED string 22. Driver circuit 10 includes an operational amplifier 14 having an output terminal coupled to a current sensitive load 16 through a DC/DC converter 18. More particularly, operational amplifier 14 has an inverting input terminal, a non-inverting input terminal, an output terminal, and an enable terminal, where the output terminal is connected to the input terminal of DC/DC converter 18, the non-inverting input terminal is coupled for receiving a source of operating potential such as, for example, a reference potential VREF, and the inverting input terminal is commonly connected to a terminal of a switch 20 and to a cathode of a diode of LED string 22. This cathode is referred to as the cathode of LED string 22. The other terminal of switch 20 is connected to a current source 24. Switch 20 has a control terminal connected to an enable terminal of operational amplifier 14 and for receiving a PWM signal. An output terminal of DC/DC converter 18 is coupled to an anode of an LED of LED string 22. This anode is referred to as the anode of LED string 22.
In operation, the PWM signal closes and opens switch 20, where closing switch 20 turns on or brightens LED string 22 and opening switch 20 dims LED string 22. When switch 20 is closed, driver circuit 10 operates in a closed loop configuration and when switch 20 is open it operates in an open loop configuration. In the closed loop configuration, current source/sink 24 sinks a constant current and LED string 20 generates a light signal. In order to sink the constant current, an adequate voltage should be delivered across current source/sink 24. This is typically achieved by sensing the voltage at the cathode terminal of LED string 22 that is connected to operational amplifier 14 and comparing it to reference voltage VREF. In response to these input signals, operational amplifier 14 generates an error signal that is applied to DC/DC converter 18 to change, i.e., to increase or decrease, its output voltage and thereby adjust the voltage at the anode terminal of the LED of LED string 22 that is connected to the output terminal of DC/DC converter 18. Adjusting the voltage at the anode terminal of LED string 22 in turn adjusts the voltage at its cathode terminal to its target value. However, using a PWM signal as the control signal results in the forward voltage of LED string 22 switching between its nominal “on” voltage and its “off” voltage, where the “off” voltage is determined by the dark current.
As discussed above, the PWM signal dims LED string 22 by opening switch 20. During the dimming phase, i.e., when the PWM signal is inactive, the feedback loop is open, the voltage at the cathode of LED string 22 is no longer valid for determining the system feedback error signal. When the PWM signal turns on again or becomes active, switch 20 closes. However, the output power from DC/DC converter 18 may not instantaneously equal the load power needed by LED string 22. FIG. 2 is a plot 30 illustrating that there is a delay after switch 20 closes and before DC/DC converter 18 becomes capable of delivering its peak output current. By way of example, the delay is 70 microseconds. The energy demanded by LED string 22 when the PWM signal becomes active exceeds the available energy from DC/DC converter 18. Thus, the eventual steady state operating conditions cannot sustain the constant current desired by LED string 22 for a PWM pulse having a duration of less than 70 microseconds. Under this condition, constant current regulation is lost and the PWM dimming becomes nonlinear. FIG. 3 is a plot 35 illustrating the energy available versus the interval of the PWM signal. In FIG. 3, the line identified by reference character 37 illustrates the energy or charge needed by DC/DC converter 18 so that it can provide sufficient energy for LED string 22. The line identified by reference character 39 is the energy available to LED string 22. For the first 70 microseconds, the energy available to LED string 22 is less than the energy needed by LED string 22. A drawback with this system is that the minimum useful PWM pulse timing is restricted, which limits the system performance. Although a DC/DC converter having a faster response may help with the system performance, it increases the cost of the system and may introduce additional problems such as unwanted Electromagnetic Interference (EMI) noise.
Accordingly, it would be advantageous to have a structure and method capable of using PWM in systems having current sensitive loads such as, for example, LED strings that are driven by a switching regulator and that do not introduce additional error sources. It would be of further advantage for the structure and method to be cost efficient to implement.